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SOFIA's 'open-heart surgery'

Credit: DLR / Clemens Plank

A rare sight – the view from inside SOFIA's telescope chamber, looking out through the telescope door opening into the Lufthansa Technik hangar

SOFIA's heart is really sensitive, which is why the doors to it are usually only opened when she is on 'Cloud 9'. At altitudes in excess of 12 kilometres, the air is very clean and there is no danger that the mirror inside SOFIA will become dirty. Any maintenance on the mirror – a thorough cleaning or its installation or removal – brings with it a high risk of damage.

At the heart of the joint NASA and DLR airborne observatory, SOFIA, is a 2.7-metre, 800-kilogram primary mirror made of fragile glass – a custom-made reflector for which there is no replacement. This is why it is only ever handled with 'velvet gloves' and treated like the princess from the fairy tale 'The Princess and the Pea'. Should the mirror break, it would be the end of SOFIA.

Yet the telescope doors have been opened on the ground?##markend##

SOFIA will be in Hamburg for a few more days for maintenance work. Last week, the aircraft structure near the telescope doors had to be inspected. To do so, heavy-duty scaffolding was set up around the delicate mirror. This meant that many people were simultaneously working very close to the mirror, which was potentially dangerous for our 'glass eye'.

Credit: DLR / Clemens Plank

The primary mirror is concealed by its red protective cover. In the middle of this is the tertiary mirror that is also surrounded by a protective cover. I am standing underneath, to give a sense of perspective.

Of course, protective measures were considered in advance in order to minimise the risk of damage. Covers for the entire mirror system are used whenever maintenance work is performed near the telescope. These covers are placed over all the mirrors' sensitive surfaces. This video of the SOFIA telescope clearly explains how the mirror system works.

For this more complex maintenance work in Hamburg, a large additional tarpaulin was manufactured to surround the entire telescope. This is a fabric cover that encloses and protects the entire mirror system, including the support structure. Air that has been cleaned using a special filter is blown under the tarpaulin. This creates positive pressure under the cover preventing dust or chemical vapours from getting near the mirrors.

Credit: DLR / Clemens Plank

Here, the white tarpaulin already covers half of the telescope. Underneath, you can clearly see the red cover over the fragile primary mirror.

Together with other safeguards, the risk can be reduced further. For example, only specially trained personnel have access to the telescope. Nevertheless, the atmosphere was extremely tense during those few days. It was a great relief when the work was successfully completed and the scaffolding could be dismantled.

Once the scaffolding was taken away and the protective cover removed, it was time for the next highlight – examination of an area of the fuselage that is concealed when the telescope doors are closed. For this rare inspection, the telescope doors had to be moved out of the way – or in other words, opened.

Credit: DLR / Clemens Plank

And vice versa – view of the telescope doors from the outside of the left side of the aircraft, looking towards the nose

Credit: DLR / Clemens Plank

An often neglected, yet equally interesting, view of the opposite (right) side. This is where the outer door moves when it opens.

All the inspections and repairs were eventually completed successfully. The entire team is pleased to have performed this 'open-heart surgery' and to have been involved in this one-of-a-kind experience. The final phase of the maintenance work is now being performed.

Next week, NASA pilots will carry out the final preparations on the aircraft before taking it for a test flight over the North Sea. If the weather is good, attentive observers will be able to see N747NA over Amsterdam, Rotterdam, Luxemburg and Frankfurt – and especially above Hannover. If all goes well, it will then finally return to sunny California the next day.

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Clemens Plank studied mechanical engineering with a focus on aerospace engineering and nuclear technology at the Technical Univeristy of Munich (TU Munich). He completed his dissertation at the Massachusetts Institute of Technology (MIT) in the United States, in which he investigated computer simulation of hydrogen deflagration.
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